Purification of sand

ABSTRACT

This invention relates to the purification of sand containing iron or iron compound impurities. The sand to be purified is contacted with concentrated hydrochloric acid which dissolves the impurities and the purified sand is freed from the unreacted acid and reaction products. This unreacted acid and the reaction products are then heated, giving hydrogen chloride gas and a decomposition product which is sent to waste. The hydrogen chloride gas is converted to concentrated hydrochloric acid which is then used to purify fresh sand.

United States Patent [72] Inventor Herbert Daymond Segrove 2,592,5804/1952 Loevenstein 23/87 X Purley, England 2,752,223 6/1956 Reeve 23/87{21] Appl. No. 856,437 2,952,516 9/1960 Gross 23/182 [22] Filed Sept. 9,1969 3,236,596 2/1966 Zirngibl etal. 23/202 [45] Patented Jan. 11,19723,399,964 9/1968 Michels et al 23/87 X [73] Assignee British IndustrialSand Limited 3,442,608 5/1969 Addinall et al. 23/154 Surrey, England 32Priority Sept. 17,1968 OTHER REFERENCES [33] Great Britain ChemicalAbstracts, 27, (1933) pg. 4047 97 [31 1 44,1 /68 Primary Examiner EdwardStern Attorney-Dowel] & Dowell [54] PURIFICATION OF SAND 10 Claims, IDrawing Fig.

[52] U.S. Cl 23/182 P, ABSTRACT; ThiS invention mates to the ifi ti fsand 23/87 R, 23/l54, 2 /2 containing iron or iron compound impurities.The sand to be [51] Int. Cl C0lb 33/12 ifi d is contacted withconcentrated hydrochloric acid [50] Field of Search ..23/182, 182 whichdissowes the impurities and the ifi d Sand is f d l, 87, 200 from theunreacted acid and reaction products. This unreacted acid and thereaction products are then heated, giving [561 References cued hydrogenchloride gas and a decomposition product which is UNITED STATES PATENTSsent to waste. The hydrogen chloride gas is converted to con- 2,031,9692/1936 McGregor et al 23/182 centrated hydrochloric acid which is thenused to purify fresh 2,155,119 4/1939 Ebner 23/1 7 sand.

Waste gases Water 1 1 Wet Hot Product sand Driersand L h Sand+ acid FetiWintersand CUiTBfliJ Heater Drum washing Dilute acid 1; Concentrated W sM gases 1 am] Q8595 Water 3 strong Acid Burner Make-up acid Ac d acidAbsorption vapour Chloride gases storage Tower Decomposition PATENTEDMPH 1 1972 EJ634332 Waste gases Water 1 2 3 Wet Hot N 4 7 Product sandDriersand Leaghjng Sand+ac|d+Feo1 sand -1: a ant,

Dlluteacld Waste g l fif gases wjater 3 1 4 AUG Burner -"9 Acid gAbsorotlon vapour Chlonoe gases storage Tower Decomposition DrgPURIFICATION OF SAND This invention relates to the purification of sandand is especially concerned with the removal of iron and iron compoundcontaminants from sand so as to give a purified sand which can be usedin the production of colorless glass.

The sand in most sand deposits contains varying amounts of iron or ironcompounds and almost invariably a substantial portion of the iron oriron compounds must be removed from the sand before it is suitable foruse in colorless glass manufacture because, even small amounts of iron,impart a green color to glass. The problem is particularly acute in theproduction of colorless glass bottles where any slight green tint tendsto be magnified by thick glass cross sections.

The demand for sand is such that the purer sand deposits are becomingworked out and so one must use sands containing larger amounts of ironand try to remove this.

The iron may be present in the sand in various forms. A certain amountmay be present in a clay formation and this can be fairly readilyremoved by a simple water washing. Other iron may be present as heavyminerals which can be removed by gravity separation or by frothflotation. Other iron may be present as a surface stain on the quartzsand grains and the removal of these stains is normally not a simplematter.

In many sands the major part of the iron impurity is in the form ofthese stains and so once the simple water washing and froth flotationpurifications have been effected there is still far too much ironpresent for the sand to be used in the production of colorless glass.

Various methods of removing these surface stains have been proposed. Onemethod is attrition cleaning in which the surfaces of the grains areabraded against one another while the sand is in the form of a slurry.Another method is to treat the sand with a dilute acid. Although someremoval of the stains can be achieved in these ways with certain sandsno really successful method has been found.

It is, therefore, an object of the invention to provide a method ofpurifying sand by the removal of these surface stains.

According to the invention there is provided a process for thepurification of sand containing iron and iron compound impurities inwhich the sand to be purified is contacted with concentratedhydrochloric acid so as to dissolve iron and iron compound impurities,after dissolution of the iron or iron compound impurities, the sandfreed from unreacted acid and the acid and sand impurity reactionproducts which consist predominately of iron chlorides, the unreactedacid and the reaction products are heated to give hydrogen chloride gasand a decomposition product which is predominately iron oxides, and theresulting hydrogen chloride gas is converted to concentratedhydrochloric acid which is recycled for use in purifying further sand.

The purification according to the invention is highly effective inremoving the surface stains from the grains of sand and in reducing theiron content of the sand to a figure well below the maximum which can betolerated in the manufacture of colorless glass. For example, sandswhich initially contain 0.13 to 0.40 percent of Fe O after being washedwith water, can be treated according to the invention to give a productcontaining 0.025 to 0.030 percent of Fe O Such a product can readily beused in the production of colorless glass. At the same time thepurification according to the invention is an economic proposition.

The hydrochloric acid is continuously recirculated and there need beonly very little wastage of it. Therefore, although it is an expensivematerial, only very small amounts of it are required once the cycle isin operation to make up losses in the cycle. Also there is no waste acidfor disposal which can often be a difficult and expensive problem. Theonly waste product is ferric oxide whose disposal is a simple matter.

The sand is purified by treatment with concentrated hydrochloric acidwhich means that the considerable agitation which would be required withdilute acid is not necessary and accordingly the apparatus in which thesand and acid are mixed can he of relatively simple and cheapconstruction. Desirably the acid has a concentration of 15 to 25 percentweight for weight. The preferred concentration is, however, about 20percent weight for weight which is approximately the composition of theconstant boiling mixture of water and hydrogen chloride at atmosphericpressure.

Before the sand is treated with the acid it has preferably been heatedand if necessary dried. The preferred temperature of the sand beforetreatment is ll0 to 120 C., but in cases where the surface stains on thesand are found to be fairly readily soluble, the temperature of the sandcan conveniently be as low as 90 C. The treatment with the acid reducesthe temperature of the sand and after the treatment the mixture isusually discharged at about to C., although the temperature may be aslow as 70 C. particularly when the sand is heated to less hightemperatures.

The invention will now be illustrated with reference to the accompanyingdiagrammatic drawing which is a flow sheet of a sand purificationprocess according to the invention.

The raw sand is normally first washed with water to remove any adheringclay and is then subjected to a preliminary conventional purification inwhich some of the iron-containing minerals are removed. Such apreliminary purification can consist of a froth flotation or a densityor magnetic separation.

The wet sand so produced is then purified in accordance with theinvention. Firstly it is dried and heated to ll 0 to [20 C. in a rotarydrum 1 by means of hot combustion gases. As an alternative to the use ofthe rotary drum 1 the sand can be dried in a fluidized bed heater.

The heated sand then passes from the drum 1 to an inclined rotaryleaching drum 2 where it is sprayed with concentrated hydrochloric acid.

The drum 2 may be lined with tiles or other refractory materials, or itmay be lined with a hardened rubber or a synthetic plastic film whichwill resist the attack of the hydrochloric acid at the temperature ofthe mixture. The length and diameter are chosen so that substantiallyall of the iron compounds staining the surfaces of the sand grains aredissolved. Generally this means that the mixture is retained in the drumfor from 30 to 90 minutes, with a preferred retention time of about 60minutes. The lining material should also be chosen so that the heatlosses are kept to a minimum. The temperature of the sand and acid atthe discharge end of the drum 2 is from 70 to 85 C. The feed anddischarge ends of the drum are sealed against vapor leaks so as to avoidlosses of hydrogen chloride vapor.

The hydrochloric acid fed to the drum 2 preferably has a concentrationof about 20 percent since if higher concentrations are used there is agreater risk of losing hydrogen chloride vapor and if a lowerconcentration is used excessive cooling of the hot sand may occur.

The quantity of acid used is normally between 2 and 12 percent by weightof the sand but about 5 percent is the preferred amount. The quantity isnot critical, however, since the acid is recovered almost completely.The use of a larger amount of acid does not therefore increase the costof the process except in so far as more heat is required to attain thereaction tem perature in the leaching drum.

In the drum 2 the acid dissolves the iron compounds which form thesurface stains and so the mixture discharged from the drum consists ofpurified sand, unreacted acid and ferric chloride.

This mixture now passes to a washing stage 3 where the mixture is washedin countercurrent with water so as to leave purified wet sand.Preferably the washing stage 3 consists of multistage filtration (fourto six stages are convenient) with washing between the filtrationstages. This ensures that the minimum quantity of washing water is usedand so reduces the amount of washings to be treated for the recovery ofhydrogen chloride. The stage 3 can consist of belt filters or tablefilters or a combination of them.

The purified wet sand from the stage 3 is taken for any furtherprocessing required, or stored either before or after having been dried.

The washings from the stage 3 consist of diluted unreacted acid andferric chloride solution, together possibly with small amounts of othermetal chlorides. The washings are then taken to a decomposition stage 4where they are heated to hydrolyze the iron chlorides back to hydrogenchloride.

The decomposition of the chlorides may be effected, for example, byspraying the washings through hot combustion gases at say 650 C., or bydropping them on to a heated rotating refractory hearth. These types ofequipment may be similar to those used for the recovery of hydrochloricacid from the spent solution used in the pickling" of steel sheet.

The gases containing the hydrogen chloride vapor pass from thedecomposition stage 4 to an absorption tower 5 where they pass incountercurrent flow to a stream of water. The hydrogen chloridedissolves in the water and the rate of flow of water to the tower 5 ischosen so that acid of the strength desired for use in the drum 2 isprovided at the base of the tower. This acid is collected and passed toa'storage tank 6 into which fresh made-up acid is also added eithercontinuously or intermittently and acid is taken from the tank 6 andrecycled to the drum 2 as required.

The ferric oxides, mainly Fe o and any other metal oxides which areproduced in the decomposition stage 4 are taken from that stage in theform of a dry powder. This material is easy to dispose of as waste sinceit is completely innocuous.

The purification according to the invention is highly effective andbecause the acid is not wasted, is an economic proposition. Also, theresulting waste products can readily be disposed of by dumping.

The purification according to the invention can be made afterpreliminary washing and other separation stages as noted above.Alternatively, the purification of the invention can be effected after awater washing and size classification and, thereafter, any dense ironmaterials not removed by the acid treatment according to the inventioncan be removed by, for example, density separation, froth flotation ormagnetic separation.

The invention will now be illustrated by the following example of aprocess according to the invention in which all proportions are byweight unless otherwise indicated.

A sand containing 0.2! percent Fe O after washing with water wasintroduced into the rotary drum 1 where it was dried and heated to 120C. At this temperature it was transferred to the leaching drum 2 whereit was mixed with hydrochloric acid of percent w/w concentration. Theacid was added in a proportion of 6 percent by weight of the sand.

The sand and acid mixture remained in the leaching drum for 60 minutesbefore it was transferred to the washing stage 3. The temperature of thesand and acid mixture at the outlet of the leaching drum 2 was 79 C. Inthe washing stage 3, the unreacted acid and metal chlorides were washedfrom the sand and the so-purified sand was found to contain 0.035percent Fe O This purified sand was then subjected to a froth flotationprocess and the resulting final product sand found to contain 0.029percent Fe o The unreacted acid and metal chlorides from the washingstage 3 were treated as described above to recover the acid.

I claim:

1. A process for the purification of sand containing iron and ironcompound impurities comprising the steps of:

a. heating said sand to be purified to a temperature of l l0 to 120 C.,

b. feeding said hot sand into an inclined rotary leaching drum, saiddrum having an upper inlet end and a lower outlet end, said sand beingfed to said upper inlet end,

c. adding concentrated hydrochloric acid to said hot sand, said acidhaving a concentration of from 15 to 25 percent weight for weight andsaid acid being mixed with said sand in a proportion of from 2 to l2percent by weight of said sand, d. rotating said drum so that said acidand sand become mixed and said acid dissolves said iron and ironcompound impurities, and said acid and sand travel from said upper inletend of said drum to said lower outlet end of said drum,

e. removing the so treated sand from said lower outlet end of said drumand freeing said so treated sand from unreacted acid and acid andimpurity reaction products which consist predominantly of iron chloridesby washing with water,

f. removing said washed sand as purified product,

g. heating said unreacted acid, reaction products and washing water togive hydrogen chloride gas and a decomposition product which ispredominately iron oxides,

h. absorbing said hydrogen chloride gas in water to give concentratedhydrochloric acid having a concentration of from 15 to 25 percent weightfor weight, and

i. recycling said acid to treat fresh heated sand in said inclinedrotary leaching drum.

2. The process of claim 1 in which said sand is heated to a temperatureof at least C.

3. The process of claim 1 in which said sand is contacted with saidconcentrated hydrochloric acid for from 30 to 90 minutes.

4. The process of claim 3 in which said sand is contacted with saidconcentrated hydrochloric acid for about 60 minutes.

5. The process of claim 1 in which said sand to be purified is contactedwith an amount of said concentrated hydrochloric acid of about 5percent.

6. The process of claim 1 in which said so treated sand is freed fromsaid unreacted acid and impurity reaction products by washing withwater.

7. The process of claim 6 in which said washing is effected by alternatefiltration and washing steps.

8. The process of claim 1 in which said unreacted acid and impurityreaction product are sprayed through hot combustion gases to give saidhydrogen chloride gas and decomposition product.

9. The process of claim 1 in which said unreacted acid and impurityreaction products are sprayed into a heated rotating refractory hearthto give said hydrogen chloride gas and decomposition product.

10. The process of claim 1 in which said hydrogen chloride gas isabsorbed in water to convert it to concentrated hydrochloric acid of thedesired strength for recycling.

2. The process of claim 1 in which said sand is heated to a temperature of at least 90* C.
 3. The process of claim 1 in which said sand is contacted with said concentrated hydrochloric acid for from 30 to 90 minutes.
 4. The process of claim 3 in which said sand is contacted with said concentrated hydrochloric acid for about 60 minutes.
 5. The process of claim 1 in which said sand to be purified is contacted with an amount of said concentrated hydrochloric acid of about 5 percent.
 6. The process of claim 1 in which said so treated sand is freed from said unreacted acid and impurity reaction products by washing with water.
 7. The process of claim 6 in which said washing is effected by alternate filtration and washing steps.
 8. The process of claim 1 in which said unreacted acid and impurity reaction product are sprayed through hot combustion gases to give said hydrogen chloride gas and decomposition product.
 9. The process of claim 1 in which said unreacted acid and impurity reaction products are sprayed into a heated rotating refractory hearth to give saId hydrogen chloride gas and decomposition product.
 10. The process of claim 1 in which said hydrogen chloride gas is absorbed in water to convert it to concentrated hydrochloric acid of the desired strength for recycling. 